This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Improvements in diode laser, fiber optic, and data acquisition technologies are enabling increased use of Raman spectroscopic techniques for both in-lab and in-situ water analysis. In this disclosure “in-situ” is used to describe a situation in which the measurement or action is or can be performed directly at the source of a sample in the field. Thus in this disclosure “in-situ” can be used interchangeably with each of “in the field”, “on-site, “in-line” and “in-flow”.
Production and environmental release of chlorinated aliphatic hydrocarbons peaked between the late eighties and early nineties. Due to their widespread use in the production of household products, degreasing operations, and petroleum refining, chlorinated solvents have become ubiquitous pollutants. According to the Environmental Protection Agency's (EPA) Toxic Chemical Release Inventory, between 1987 and 1993, releases to water and land of trichloroethylene (TCE), 1,2-Dichloroethane (1,2-DCA), Dichloromethane (DCM), and 1,1,1-33 Trichloroethane (1,1,1-TCA) totaled over 1.7 million kg, representing only a subset of all chlorinated solvents thus released. These contaminants are still known to persist in the environment and it has been reported that up to 34% of the drinking water supply sources in the U.S. likely contain TCE contamination. Compounds such as trichloroethylene (TCE) and tetrachloroethene (PCE) are found at approximately 80% of all Superfund sites with groundwater contamination (852 facilities) and more than 3000 Department of Defense (DoD) sites in the U.S. The net result is that these solvents pose a considerable threat as groundwater contaminants and account for a significant proportion of the more than 90 contaminants of drinking water listed by the EPA.
Chlorinated solvents are Dense Non-Aqueous Phase Liquids (DNAPLs) and are denser than water and thus when spilled or released into the environment tend to migrate downward in the subsurface. Their migration through the vadose zone can lead to residual pockets of contaminants in soil pore spaces (source zones), the release of vapors in soil pore space, dissolution in groundwater (to the extent possible) in the capillary fringe and below the groundwater table, and pooling above an aquitard, as well as continued infiltration into fractures in boundary rock layers. The tendency for these compounds to partition (i.e. capable of going into solution) makes them exceptionally menacing pollutants. While the fate and transport of the dissolved fractions are influenced by phenomenon such as groundwater advection, mechanical dispersion, molecular diffusion, and groundwater-porous media chemical partitioning, movement of the compounds can be significantly influenced by gravity and capillary effects that make it difficult to adequately characterize and track their presence in the subsurface. The sporadic and localized concentration of chlorinated solvents in regions such as soil pores and retarded pools also serves as a recalcitrant contaminant source which can often frustrate remediation attempts.
The unique challenges posed by chlorinated solvent DNAPLs in the geoenvironment have led both the U.S. EPA and the National Academy of Sciences to conclude that DNAPL sources may be contained, but remediation to typical cleanup levels for most DNAPL sites is often “technically impracticable” at justifiable costs This realization has driven a shift in the strategies pursued to deal with sites contaminated with chlorinated solvents. Most notably, added attention has been given to the potential to pursue “monitored natural attenuation” (MNA) rather than or in combination with proactive remediation. This site management approach relies upon natural physical, chemical and biological processes to reduce contaminant concentrations. In this context, application of mass balance principles must demonstrate that the summation of the effects of available natural attenuation mechanisms will be sufficient to remediate the site and protect vulnerable interests. Successful application of MNA typically requires extensive site analyses for initial screening for MNA potential, verification of attenuation processes, confirmation of effective MNA, and long term monitoring for any changes to the natural system that could alter the potential to achieve remediation goals. While initially conceived as a cost effective alternative to proactive remedial intervention it is becoming clear that the costs of site characterization and long term monitoring may be prohibitive, and so significant effort has been focused on developing more cost-effective approaches to carry out these tasks.
Currently, the technology used in common practice to assess chlorinated solvent levels in a contaminated site with actionable reliability involves either very costly and/or sophisticated laboratory instrumentation or expensive field instruments that can assess only a limited set of compounds, provide information over a limited spatial extent, and/or are too costly or complex to lend themselves to continuous long-term monitoring scenarios. Attempts to measure the effects of remediation efforts or the availability of attenuation mechanisms, for example to support mass balance evaluations of monitored natural attenuation, are even more challenging. The challenges are due to associated complexities such as inaccessibility of ground environment and simultaneous action of multiple attenuation mechanisms.
In the realm of laboratory measurements of chlorinated solvents and their daughter compounds, a host of wet chemistry techniques are applied. Chief among these techniques are: variants of gas chromatography-mass spectrometry (GC-MS) that have been adapted to perform stable isotope analysis of carbon and chlorine to assess levels of chlorinated aliphatic hydrocarbons while mitigating alteration of sample components resulting from pre-processing (e.g., Helium Microwave Induced Plasma MS to GC); and, carrier gas extraction to GC multiple collector inductively coupled plasma-source mass spectrometry (MC-ICPMS). While these techniques require field acquisition of samples (e.g., with devices such as the Waterloo profiler or precision injection/extraction probes and subsequent laboratory preparation and analysis which is are time consuming and expensive processes, they yield highly chemical-specific results with great sensitivity (ppb or better).
In the field, remote sensing techniques (primarily satellite based) provide general information on field conditions or inferential indications of contaminant presence (such as foliage discoloration or depletion), yet offer limited resolution (typically several square meters), and provide virtually no insight into subsurface conditions. Geophysical methods, such as electrical resistivity imaging and non-linear complex-resistivity cross-hole imaging, can successfully indicate the presence of chlorinated solvents in-situ, but typically at limited depths, spatial resolution, and sensitivity. Concepts that involve the introduction of foreign materials, such as graphite, Zeolite, or Samms particles, into the subsurface to enhance geophysical signatures have been proposed. Portable GC/MS units have also emerged commercially, but again require considerable sample handling. Other novel field approaches to chlorinated solvent evaluation include single point sensors that use replenishable reagents or biosensors but any one sensor of this type tends to have limited versatility. Beyond these techniques, an array of indirect methods are employed to indicate the presence of DNAPLs such as chlorinated solvents including use of reactive inter-well tracers, radon flux sensors, soil vapor probes, and membrane interface probes that monitor related volatiles However, these in-situ techniques tend to provide only directional input on the presence of contaminants or provide information over a very limited spatial extent.
In other cases, sensors are incorporated into cone penetrometer-type devices, and used to make direct field measurements. These instruments provide flexibility by enabling the investigator to rapidly examine multiple points in the field, both spatially and at depth, while limiting the errors that can often accompany sample extraction techniques, and thus their use has become increasingly common in practice. The majority of these direct penetrometer-based systems make use of optical spectroscopic phenomenon. Near-infrared (NIR) probes have been explored, as well as optical fiber sensors that rely on evanescent field phenomenon. However, these devices by their very nature apply only to compounds which absorb at the frequency of the system excitation source. Ultra-violet (UV) Laser Induced Fluorescence has also been explored, but tends to have difficulty yielding chemical specific or quantitative insights due to the overlap of fluorescence signatures of similar compounds. Some of the most promising results in this arena stem from work done with continuous wave (CW) Raman spectroscopy which has been shown to yield highly specific chemical signatures even in complex in-situ settings. Effectiveness of Raman systems, however, has traditionally been adversely affected by fluorescence interference in natural environments. More recently demonstrated is the ability to significantly suppress the impact of fluorescence on Raman observations by employing pulsed-laser technology to perform Time Resolved Raman Spectroscopy and effectively gate Raman from fluorescence phenomenon in the time-domain, with a closed-path fiber optic system that could be incorporated in a penetrometer or in a field monitoring station.
Despite these advances, in-situ analyses, which are necessary for cost effective site management, inevitably involve tradeoffs between the time and cost associated with the analysis, and the degree to which the results represent actual field conditions and can provide information of actionable quality. In particular, one of the biggest challenges with arguably the most versatile of the optical techniques, Raman spectroscopy, is its overall sensitivity
In some studies, direct Raman analysis was performed on samples of Ottawa sand and two NIST (National Institute of Standards and Testing) soils, saturated with neat TCE, using the 488 nm line of a 100 mW Argon laser. In these studies the researchers reported clear evidence of the C—Cl vibrational mode at 628 cm-1, but the intensity of the Raman line did not correlate well to the soil mass fraction of the compound, indicating presence but providing little ability to quantify in-situ concentrations. In 1999, the USDOE (US Department of Energy) reported on the second generation of a cone-penetrometer equipped with a 785 nm CW Raman system (originally a 415 nm system) which was employed in two distinct in-situ settings to a) evaluate DNAPL contaminated radioactive waste in large containment vessels and b) assess DNAPL contamination directly in soils. For the in-tank tests, “the Raman probe demonstration resulted in more than 99 percent accuracy for compound identification and greater than 93 percent accuracy in identifying both the compounds and the concentration” for organic constituents above 20% by weight. For the direct push soil tests, the Raman probe was reported to be effective at detecting PCE at the “highest levels of contamination” in saturated zones (˜1,500 ppm by mass), and was less reliable in the vadose zone. Later work made use of a 300 mW 785 nm CW laser and successfully detected the presence of TCE in-situ in proximity to a solvent storage tank at locations with depth that were later verified in the laboratory to contain concentrations of 200-750 ppm by mass TCE, but failed to detect TCE in other zones with concentrations on the order of 50 ppm by mass. No major advances in in-situ Raman analysis of chlorinated solvents have been reported for the last decade.
These initial studies helped define the value of even coarse in-situ Raman analysis and several commercially available cone-penetrometer Raman probe systems now exist. However, to date, these probes are generally used for rapid detect-non-detect profiling as detection limits have been high relative to chlorinated solvent concentrations typically present in the field. This is due to an inability to manage interference in the field and separate that interference from desired observations and the fact that the systems have traditionally used low resolution CCD detectors, an issue that limits a system's ability to differentiate two varying analyte concentrations.
Sites contaminated with chlorinated solvents frequently contain the pollutant at levels ranging from a neat state to low ppb concentrations that exceed Maximum Contaminant Level (MCL) drinking water standards (e.g., TCE MCL=5 ppb), with aqueous concentrations of 3 to 500 ppm commonly encountered in groundwater. By comparison, the detection levels reported in the historical studies noted above, which are on a mass of soil basis, are equivalent to ˜300 to 9000 ppm on a volume to volume basis in the pore fluid of a soil (assuming a typical range of soil void ratios from 0.3 to 1.2 for the soils studied). To this end, any improvements to the sensitivity and resolution of a monitoring technology can be very valuable to facilitate enhanced plume delineation, earlier warnings of contaminant release from a containment zone, and/or the design of more tailored monitoring approaches.
Due to the factors mentioned above, a need exists for a method and apparatus to improve the overall sensitivity of Raman observations of chlorinated solvents through indirect measurements that take advantage of the influence of chlorine on the vibrational modes of water to indicate the presence of chlorinated compounds in solution.